Fishmen were bipedal and physically strong. They were dark green, scaly, and had extremely large eyes. They had flipper-like feet with three long toes adapted for both swimming and walking. Some wore tunics and red sashes.
Fishmen were highly inventive. They were able to create a variety of materials which were either available naturally or synthesized out of the dense liquid of their world. They built massive, drifting underwater cities surrounded by spherical self-sealing membranes. They also built submarines and tanks. Hand-held weapons included a webb-gun that fired weighted nets and a freeze gun that encased its target in ice.
In 2265, when the USS Enterprise crashed into the globular planet, most systems were disabled by the unknown liquid. The ship was spotted by a group of fishmen, and soon a sleek submarine arrived to tow the Enterprise to the edge of a vast city. Small groups of fishmen emerged carrying tools and weapons to try to force open the hull. James T. Kirk and Spock donned pressure suits and used their translator communicator to make first contact. "Earth" and "space" were unrecognized terms, so a swarm of fishmen decided to bring the strange intruders to one of their scientists in the city for dissection and study.
Kirk escaped and promptly prepared a rescue squad. Since phasers did not function in the dense liquid ocean, they fought against the fishmen with knives, spears and cudgels. The fishmen retreated and summoned a tank. Needing to capture its freeze gun cannon, the rescue party led the tank forward, while Kirk and Spock attacked it from the rear and subdued its pilots. With the cannon, they enveloped the Enterprise in ice, lightening the ship enough for it to float to the surface, where it could overcome the effects of the liquid and escape back to space. (TOS comic: "Nor Any Drop to Drink")